1. Field of the Invention
The present invention relates to a coating substrate for thin-film coatings used in the manufacture of electronic components, said coating substrate containing a rolled product of aluminium or aluminium alloys, and being such that the substrate surface to be coated exhibits local irregularities in the structure in the form of small holes or small-grained or needle-shaped elements in the substrate surface. Further, the present invention relates to a process for manufacturing the coating substrate. In addition, the use of the coating substrate according to the invention is described.
2. Discussion of the Prior Art
The term electronic components includes e.g. electronic thin-film components such as diodes or transistors, in particular solar cells. The economic use of such electronic components depends essentially on the cost of manufacturing them.
By using thin-film technology, materials savings can be made in the manufacture of such electronic components and, by using continuous coating methods, large quantities can be made at favourable cost. As far as solar cell manufacture is concerned, this involves essentially solar cells of amorphous silicon (a-Si), or microcrystalline silicon (.mu.c-Si), or multiple cells of both or one of the two mentioned materials. Compared with solar cells of crystalline silicon which are made in batch processes e.g. on silicon wafers, such solar cells exhibit lower efficiency, but the cost of manufacture is much more favourable than that of solar cells made of crystalline silicon. Because of the lower efficiency of solar cells made from a-Si or .mu.c-Si--compared with solar cells made from crystalline silicon--large area, cost favourable modules are necessary in order to be able to use them economically.
The production of solar cells of a-Si or .mu.c-Si on glass substrates or foils of stainless steel is known.
Extended Abstracts, Vol. 86, No. 1, May 1986, p. 59 describes the manufacture of an a-Si solar cell of high purity (99.99 wt. %) in which the aluminium substrates are cleaned chemically and a repetitive surface structure is etched into the surface layer of the aluminium substrate by means of electrochemical methods in order to achieve multiple reflection of the incident light.
The publication EP-A-O 155 758 describes substrates with a light-sensetive composite layer containing a-Si for digital imaging using coherent laser-beam light in which, in order to avoid interference due to variations in layer thickness, the composite layer is made up in such a manner that at least one pair of non-parallel interfaces is produced between the layers, so that a layer with a continuously changing layer thickness is produced between these interfaces.
Up to now--because of the much higher efficiency, and hence smaller surface area required mono or polycrystalline silicon solar cells have been almost exclusively employed for photovoltaic energy-producing units that have been installed individually e.g. mounted on roofs or as free-standing units.
However, solar cells of a-Si or .mu.c-Si exhibit large potential in integral photovoltaic units for which a support surface is already provided for the solar cells. Especially in the construction of photovoltaic facade elements solar cell modules of a-Si or .mu.c-Si can be deposited over large areas directly onto the facade element using plasma-deposition, CVD (chemical vapour deposition) and PVD (physical vapour deposition) processes. As a result, a photovoltaic module of a-Si or .mu.c-Si deposited directly onto a facade element requires only approximately a quarter of the primary energy necessary to manufacture a corresponding module of crystalline silicon.
The potential for large area solar cells of a-Si or .mu.c-Si is therefore enormous. Such solar cells can be employed in buildings e.g. as facade elements, or in vehicle manufacture e.g. as bodywork cladding, and contribute significantly to the generation of electricity.
However, the production of electronic components using thin-film technology requires a suitable substrate to be made available for that purpose. In particular, the production of large area photovoltaic solar cells requires cost-favourable substrates with surface properties that are adequate for thin-film coating purposes.
For reasons of costs and weight, large area aluminium panels or composite panels with aluminium outer layers are widely used today. For that reason, depositing solar cells on substrates of aluminium or aluminium alloys by thin-film technology could strongly favour the economic exploitation of solar energy via photovoltaic units.
A significant additional advantage to be obtained from the use of aluminium coating substrates is the ductility of aluminium and its alloys. The ductility of that material permits easy rolling and therefore cost-favourable manufacture of large area substrates and, also allows the substrate surface to be embossed, in particular by rolling. The embossing enables optimised surface structures to be made for specific purposes e.g. inverted pyramids or saw tooth structures in the sub-micron to millimeter range, for example for photovoltaic applications.
Known from the field of semiconductor technology is that aluminium and silicon can react on coming into direct contact with each other, various reactions taking place already below 200.degree. C. The use of aluminium substrates in thin film technology usually requires therefore the deposition of a diffusion barrier layer and/or insulating layer. In order to prevent the interdiffusion of aluminium and silicon, up to now various effective, but complicated and expensive diffusion barrier layers have been made in tests. For large area and cost favourable photovoltaic modules, however, diffusion barrier layers out of complicated multilayer systems have to be avoided. Also such a diffusion barrier layer should not involve the incorporation of any expensive materials. A further requirement regarding diffusion barriers concerns the compatibility of the method of their manufacture and materials with the substrate body material and the production of the substrate.
The use of aluminium as a substrate material for thin film solar cells of a-Si or .mu.c-Si is described in DE 35 28 087, the substrate surface being given an oxide layer as barrier and insulating layer in the form by anodising in oxalic acid. In that case aluminium substrates of high purity are necessary to produce the anodic oxide layer.
Especially when depositing very thin layers in the gas phase, e.g. plasma deposition of a-Si or .mu.c-Si, it has been found that surfaces, e.g. aluminium substrates featuring an anodic oxide layer exhibit surfaces, in which interdiffusion of aluminium and silicon occurs and/or exhibit locally defective sites which lead to short circuiting, can lead to a breakdown in the whole thin layer module. The above mentioned problems appear especially when employing commercially available aluminium substrates out of pure aluminium or aluminium alloys.
The economic manufacture of thin film semiconductor elements normally requires the silicon layers to be deposited by plasma deposition as this enables layers to be produced at low substrate temperatures. By way of contrast, other thin film solar cell technologies such as e.g. CIS (copper indium diselenide) or CIGS (copper indium gallium diselenide) are unsuitable for aluminium substrates as they require temperatures close to the melting point of aluminium. Although plasma deposition takes place at low substrate temperatures, the substrate surface can become heated very locally at specific sites (grain or needle-shaped surface structures which make poor thermal contact with the substrate) leading, as a result, to plasma induced interdiffusion of e.g. aluminium and silicon. Thereby it should be noted that the temperature of the substrate surface is not, or only insignificantly, increased by the plasma deposition process, as an increase in the kinetic energy of some surface atoms as a result of ion bombardment can not be regarded as an increase in temperature in the classical sense.